bug#28611: 26.0.60; iconifying a child frame seems to be problematic

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Mon, 02 Oct 2017 13:27:52 -0400
> Cc: 28611@debbugs.gnu.org
> 
> b03b4f6d7 causes without-x builds to fail:
> 
> frame.c:2545:25: error: 'struct frame' has no member named 'parent_frame'
>    Lisp_Object parent = f->parent_frame;

Thanks, fixed.